1. Field of the Invention
The present invention relates to a both-side light emitting organic electroluminescence display device, more particularly, to a both-side light emitting organic electroluminescence display device including a reflection film arranged under a first electrode.
2. Description of Related Art
Generally, an organic electroluminescence (EL) display device is a self-emissive flat panel display device that emits light by electrically exciting an organic compound. The organic EL display device may be fabricated at a low temperature, it has a high response speed of 1 ms or less, it has low power consumption, it has a wide viewing angle due to its self-emissive characteristics, and it has high contrast. It may be thin and lightweight since a backlight, which is used in other products such as a liquid crystal display (LCD) is not necessary, and it may be used in products such as cellular phones, etc. due to its uniform surface light emitting characteristics and fabrication convenience.
An organic EL display device includes an organic light-emitting layer between an anode and a cathode. Holes from the anode combine with electrons from a cathode in the organic light-emitting layer to form excitons, which are hole-electron pairs, and the organic light-emitting layer emits light when the excitons transition from an excited to ground state.
Generally, an organic EL display device may be a passive matrix or an active matrix device according to the method for driving N X M pixels arranged in a matrix. Passive matrix organic EL display devices are typically used for small, low resolution displays since a display region of the passive matrix device is constructed in a simple matrix of anodes and cathodes. On the other hand, an active matrix organic EL display device includes at least two thin film transistors per each pixel. Hence, the active matrix display is capable of exhibiting more stable luminance by supplying a constant electric current irrespective of the number of pixels, and it is typically used for large, high resolution displays due to its low power consumption.
Furthermore, organic EL display devices may be bottom light-emitting and top light-emitting devices according to the direction light is emitted from an organic light-emitting layer. The bottom light-emitting device emits light to the side of a substrate on which the device is formed, a reflection electrode is formed on an upper part of the organic light-emitting layer, and a transparent electrode is formed on a lower part of the organic light-emitting layer. An active matrix organic light-emitting device may have a reduced light emitting area since the area in each pixel where the thin film transistors are formed is a non-light emitting area. On the other hand, luminance of the top light-emitting organic EL display device is improved since its light transmission area may be increased by forming a transparent electrode on an upper part of an organic light-emitting layer and forming a reflection electrode on a lower part of the organic light-emitting layer, such that light emits in a direction opposite to the substrate. Currently, a both-side light emitting organic EL display device, which is capable of simultaneously realizing top light emission and bottom light emission on one substrate, is being noticed as a next generation flat panel display device.
FIG. 1 is a plan view of a conventional both-side light emitting organic EL display device.
Referring to FIG. 1, the conventional both-side light emitting organic EL display device includes a first transparent electrode 110 and a pixel defining layer 130 formed on the first electrode 110 to expose the first electrode. Light may be emitted to front and rear sides of the device from a part where the first electrode 110 is formed by sequentially forming an organic film layer, which includes at least an organic light-emitting layer, and a second transparent electrode on the first electrode 110.
The both-side light emitting organic EL display device and a fabrication method thereof are described in detail below with reference to FIG. 2 and FIG. 3.
FIG. 2 is a cross sectional view along line I-I′ of FIG. 1 in a conventional both-side light emitting organic EL display device, and FIG. 3 is a cross sectional view for explaining a conventional both-side light emitting organic EL display device and a method for fabricating the conventional both-side light emitting organic EL display device.
Referring to FIG. 2, the first electrode 110 is patterned on a substrate 100 formed of glass or plastic. The first electrode 110 is a transparent electrode that may be formed of indium tin oxide (ITO) or indium zinc oxide (IZO), which have a high work function, when the first electrode 110 is an anode. When the first electrode 110 is a cathode, it may be formed as a transmission electrode formed of a material selected from the group consisting of Mg, Ca, Al, Ag and an alloy thereof as a conductive metal having a low work function.
A pixel defining layer 130 for partially exposing the surface of the first electrode may then be formed on the first electrode 110.
FIG. 3 is a cross sectional view of the conventional both-side light emitting organic EL display device of FIG. 2 in which an organic film layer and a second electrode are additionally formed on the exposed first electrode and the pixel defining layer.
Referring to FIG. 3, an organic film layer 140 including at least an organic light-emitting layer is formed on the exposed first electrode 10. In addition to the organic light-emitting layer, the organic film layer 140 may include one or more of the following layers: a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer.
Next, a second electrode 150 is formed on the organic film layer 140 over the entire substrate. The second electrode 150 may be a transmission electrode formed of a material selected from the group consisting of Mg, Ca, Al, Ag and an alloy thereof as a conductive metal having a low work function when the first electrode 10 is a transparent anode, and the second electrode 150 may be a transparent electrode formed of ITO or IZO when the first electrode 110 is a cathode.
A conventional both-side light emitting organic EL display device may emit light only from a part where the first electrode is formed. Hence, a small amount of the generated light may be emitted, and life of the device may be shortened accordingly since a part where the first electrode is formed may have a small area.